Conventionally, a four-axis lathe including two spindle heads and two tool posts is widely used for a more effective machining of a workpiece. In the four-axis lathe, a turning is carried out by a chuck of one spindle head, and when that machining is finished, the workpiece is clamped by a chuck of another spindle head and a turning of the surface opposite to the machined side is carried out.
A brief description of the prior art is now given, with reference to the attached drawings. FIGS. 4 (a), (b) and (c) are respectively diagrams illustrating the exchanging the workpiece in a CNC lathe having two spindle heads. FIG. 4 (a) shows an outline of a workpiece exchanging operation. In the FIG. 4 a chuck A2 is mounted at a spindle head A1 and is clamping the workpiece 5, and when the machining thereof by a tool post A3 is completed, a spindle head B6 is moved close to the workpiece 5, and the workpiece 5 is clamped by a chuck B7 mounted at the spindle head B6. At that time, the chuck A2 is opened, the spindle head B6 is returned to the machining position, and the workpiece 5 is machined by a tool post 8. FIG. 4 (b) shows the state wherein the workpiece 5 is normally clamped by the chuck B7 of the spindle head B6, and FIG. 4 (c) shows the state wherein the workpiece 5 is not normally clamped by the chuck B7 of the spindle head B6.
According to a workpiece exchanging system in a conventional CNC lathe having two spindle heads a clearance exists between the workpiece and the chuck. For example, when a foreign substance has intruded between the workpiece and the chuck, the workpiece is again machined, and as a result, the finished workpiece is not accurately machined to a desired dimension. In addition, on occasion, problems arise such that the workpiece is not correctly clamped by the chuck, and thus falls out during the machining, or the tool and the workpiece come into violent contact due to a quick feeding thereof.